Equilibrium and kinetic modeling of the adsorption of stevioside on amberlite XAD-7 beads

Abstract

The behaviour of the stevioside adsorption process from the stevia extract solutions onto XAD-7 beads was studied in a batch system. The equilibrium isotherms and the kinetics of adsorption with respect to the initial stevioside concentration (370, 667,1185, and 1956 μg/ml), solution temperature (25°C, 55°C, and 75°C) and weight of resin used (0.1g, 0.3g and 0.5g) were investigated at pH 9. Langmuir and Freundlich adsorption models were tested to describe the experimental isotherms and isotherm constants. The equilibrium data was found to fit well to the Langmuir model in the saturation concentration range from 0 to 820 μg/ml. The maximum monolayer adsorption capacities obtained from Langmuir model were 398, 385 and 270 mg/g for 0.1 g XAD-7 beads used, respectively at temperature 25°C, 55°C, and 75°C and pH 9. The pseudo first-order and second order kinetics models were used to describe the kinetic data and the rate constants were also evaluated. The experimental data fitted well to the second-order kinetic model. The initial stevioside concentration and the weight of resins used both significantly affect the adsorption capacity but the temperature and the solution pH were found to be relatively minor factors. Adsorption capacity increases with the increase of initial stevioside concentration, and solution pH as well as the decrease of the solution temperature, and resin weight used. The activation energy is 20kJ/mol for the adsorption of stevioside on 0.1g XAD-7 beads used at pH 9 with an initial stevioside concentration of 370 μg/ml.